Grandmothering life histories and human pair bonding.

The evolution of distinctively human life history and social organization is generally attributed to paternal provisioning based on pair bonds. Here we develop an alternative argument that connects the evolution of human pair bonds to the male-biased mating sex ratios that accompanied the evolution of human life history. We simulate an agent-based model of the grandmother hypothesis, compare simulated sex ratios to data on great apes and human hunter-gatherers, and note associations between a preponderance of males and mate guarding across taxa. Then we explore a recent model that highlights the importance of mating sex ratios for differences between birds and mammals and conclude that lessons for human evolution cannot ignore mammalian reproductive constraints. In contradiction to our claim that male-biased sex ratios are characteristically human, female-biased ratios are reported in some populations. We consider the likelihood that fertile men are undercounted and conclude that the mate-guarding hypothesis for human pair bonds gains strength from explicit links with our grandmothering life history.

Grandmothering drives the evolution of longevity in a probabilistic model.

We present a mathematical model based on the Grandmother Hypothesis to simulate how human post-menopausal longevity could have evolved as ancestral grandmothers began to assist the reproductive success of younger females by provisioning grandchildren. Grandmothers׳ help would allow mothers to give birth to subsequent offspring sooner without risking the survival of existing offspring. Our model is an agent-based model (ABM), in which the population evolves according to probabilistic rules governing interactions among individuals. The model is formulated according to the Gillespie algorithm of determining the times to next events. Grandmother effects drive the population from an equilibrium representing a great-ape-like average adult lifespan in the lower twenties to a new equilibrium with a human-like average adult lifespan in the lower forties. The stochasticity of the ABM allows the possible coexistence of two locally-stable equilibria, corresponding to great-ape-like and human-like lifespans. Populations with grandmothering that escape the ancestral condition then shift to human-like lifespan, but the transition takes longer than previous models (Kim et al., 2012). Our simulations are consistent with the possibility that distinctive longevity is a feature of genus Homo that long antedated the appearance of our species.

Blood cell telomere lengths and shortening rates of chimpanzee and human females.

OBJECTIVES: Slower rates of aging distinguish humans from our nearest living cousins. Chimpanzees rarely survive their forties while large fractions of women are postmenopausal even in high-mortality hunter-gatherer populations. Cellular and molecular mechanisms for these somatic aging differences remain to be identified, though telomeres might play a role. To find out, we compared telomere lengths across age-matched samples of female chimpanzees and women. METHODS: We used a monochrome multiplex quantitative polymerase chain reaction to assay canonical telomere repeats in blood cells from captive female chimpanzees (65 individuals; age: 6.2-56.7 years) and compared them to the same measure in human females (43 individuals; age: 7.4-57.3 years). RESULTS: Our samples showed little difference in attrition rates between the species (~0.022 T/S per year for chimpanzees and ~0.012 T/S per year for humans with overlapping 95% confidence intervals), but telomeres were twice as long in chimpanzees as in humans (T/S ratios = 2.70 and 1.26, respectively). CONCLUSIONS: Based on the longevity differences, we initially hypothesized that telomere shortening rates would be faster in chimpanzees than in humans. Instead, it is shorter telomere length that appears to be the derived state in humans. This comparison indicates that better characterization of physiological aging in our closest living relatives will be indispensable for understanding the evolution of distinctive human longevity.

Grandmothers and the evolution of human longevity: a review of findings and future directions.

Women and female great apes both continue giving birth into their forties, but not beyond. However humans live much longer than other apes do. Even in hunting and gathering societies, where the mortality rate is high, adult life spans average twice those of chimpanzees, which become decrepit during their fertile years and rarely survive them. Since women usually remain healthy through and beyond childbearing age, human communities include substantial proportions of economically productive postmenopausal women. A grandmother hypothesis(8-12) may explain why greater longevity evolved in our lineage while female fertility still ends at ancestral ages. This hypothesis has implications for the evolution of a wide array of human features. Here we review some history of the hypothesis, recent findings, and questions for ongoing research.

Brief communication: Adrenal androgens and aging: Female chimpanzees (Pan troglodytes) compared with women.

Ovarian cycling continues to similar ages in women and chimpanzees yet our nearest living cousins become decrepit during their fertile years and rarely outlive them. Given the importance of estrogen in maintaining physiological systems aside from fertility, similar ovarian aging in humans and chimpanzees combined with somatic aging differences indicates an important role for nonovarian estrogen. Consistent with this framework, researchers have nominated the adrenal androgen dehydroepiandrosterone (DHEA) and its sulfate (DHEAS), which can be peripherally converted to estrogen, as a biomarker of aging in humans and other primates. Faster decline in production of this steroid with age in chimpanzees could help explain somatic aging differences. Here, we report circulating levels of DHEAS in captive female chimpanzees and compare them with published levels in women. Instead of faster, the decline is slower in chimpanzees, but from a much lower peak. Levels reported for other great apes are lower still. These results point away from slowed decline but toward increased DHEAS production as one of the mechanisms underlying the evolution of human longevity.

Increased longevity evolves from grandmothering.

Postmenopausal longevity may have evolved in our lineage when ancestral grandmothers subsidized their daughters' fertility by provisioning grandchildren, but the verbal hypothesis has lacked mathematical support until now. Here, we present a formal simulation in which life spans similar to those of modern chimpanzees lengthen into the modern human range as a consequence of grandmother effects. Greater longevity raises the chance of living through the fertile years but is opposed by costs that differ for the sexes. Our grandmother assumptions are restrictive. Only females who are no longer fertile themselves are eligible, and female fertility extends to age 45 years. Initially, there are very few eligible grandmothers and effects are small. Grandmothers can support only one dependent at a time and do not care selectively for their daughters' offspring. They must take the oldest juveniles still relying on mothers; and infants under the age of 2 years are never eligible for subsidy. Our model includes no assumptions about brains, learning or pair bonds. Grandmother effects alone are sufficient to propel the doubling of life spans in less than sixty thousand years.

Parental attitudes, beliefs, and perceptions about genetic testing for FAP and colorectal cancer surveillance in minors.

Familial adenomatous polyposis (FAP) is the second most common hereditary colorectal cancer syndrome and confers a nearly 100% lifetime risk of developing colorectal cancer. Understanding factors that facilitate and inhibit genetic testing and cancer surveillance in children who are members of families affected by FAP will better equip clinicians to clarify misunderstandings and facilitate appropriate care. The aims of this study were to examine parental attitudes and beliefs regarding endoscopic surveillance and genetic testing in minors at risk for developing FAP. This cross-sectional study includes analyses of qualitative and quantitative interview data collected from parents of children with or at risk for FAP. This report includes data from 28 parents with a total of 51 biological children between 10-17 years of age. The parents had a clinical and/or genetic diagnosis of FAP. Most commonly reported facilitators included provider recommendation (surveillance) and personalized medical management (genetic testing). Most commonly reported barriers included lack of provider recommendation (surveillance) and cost (genetic testing).

Religiosity, spirituality, and psychological distress in African-Americans at risk for having a hereditary cancer predisposing gene mutation.

Elevated psychological distress has been observed among people at increased risk for familial cancer. Researchers consider religiosity and spirituality (RS) to be positive coping mechanisms associated with reduced psychological distress. Relatively little is known about the impact of RS on genomic health issues. The objectives of our study were: (1) describe the prevalence of RS and depressive symptoms and (2) explore how RS relates to psychological distress in a cohort of individuals with a > or =25% prior probability of a genetic predisposition to cancer. Participants (n = 99) were drawn from an African-American, Louisiana-based kindred with a mutation at the BRCA1 locus. This analysis reports findings from a survey assessing RS and the use of three types of religious coping styles: collaborative, self-directing, and deferring. Clinically significant depressive symptoms were relatively high (27%); with females (33%) more likely than males (17%) to report symptoms (P < 0.01). The majority of participants reported being highly religious. The most commonly employed religious problem solving style used by participants was collaborative (X=22.9; SD=5.8) versus self-directing (X=12.8; SD = 5.1) and deferring (X=19.9; SD = 6.3). We did not observe significant associations between RS indicators and psychological distress, nor did we observe appreciable differences related to gender or risk perception. Although RS beliefs and practices are important for many African-Americans, we did not find evidence that indicators of self-reported RS are associated with psychological distress prior to genetic counseling and testing.

Age-related decline in ovarian follicle stocks differ between chimpanzees (Pan troglodytes) and humans.

Similarity in oldest parturitions in humans and great apes suggests that we maintain ancestral rates of ovarian aging. Consistent with that hypothesis, previous counts of primordial follicles in postmortem ovarian sections from chimpanzees (Pan troglodytes) showed follicle stock decline at the same rate that human stocks decline across the same ages. Here, we correct that finding with a chimpanzee sample more than three times larger than the previous one, which also allows comparison into older ages. Analyses show depletion rates similar until about age 35, but after 35, the human counts continue to fall with age, while the change is much less steep in chimpanzees. This difference implicates likely effects on ovarian dynamics from other physiological systems that are senescing at different rates, and, potentially, different perimenopausal experience for chimpanzees and humans.